1. Field of the Invention
The present invention relates to a resin for toner contained in a toner used in an image forming method utilizing electrophotography or electrostatic printing or toner jet image forming method. In particular, the present invention relates to a resin for toner contained in a toner to be provided in a fixing method that involves fixing a toner image formed by the toner on a print sheet such as a transfer material under heat and pressure. Further, the present invention relates to a toner that contains the resin for toner.
2. Related Background Art
Conventionally, the image forming process utilizing electrophotography, electrostatic printing and so on is constructed such that an electrostatic latent image on a photosensitive drum is developed by electrostatic force depending on a potential difference on the photosensitive drum. In this case, toners are charged by friction between toners or between a toner and a carrier, and further due to friction with a control blade. Therefore, it is indispensable to control the chargeability of toner in addition to control of toner particle diameter and particle diameter distribution and so on.
To control the chargeability of the toner, the triboelectric property of the binder resin itself may be utilized. However, binder resins commonly used for toners have low triboelectric properties so that it is difficult to control the chargeability by the composition of the binder resin. Accordingly, generally, a charge control agent is added in order to impart chargeability to the toner.
Conventionally, negatively chargeable charge control agents include metal complex salts of monoazo dyes; nitrohumic acid and salts thereof; metal compounds, boron compounds, urea compounds and silicon compounds of salicylic acid, alkylsalicylic acids, dialkylsalicylic acids, naphthoic acid, dicarboxylic acids and so on; calixarenes; sulfonated copper phthalocyanine pigments; chlorinated paraffins, and so on. Those charge control agents have complex structures and are variable in their property and many of them have poor stability; in particular, most of them vary in chargeability depending on their environment such as temperature and humidity. Also, some of them are denatured due to decomposition or the like upon kneading with heating.
Further, the charge control agent to be added to the toners must be present on the surface of the toner in a certain amount to impart triboelectric chargeability to the toner. For this reason, the friction between the toners, collision of the toner with the carrier, friction of the toner with a conveyor sleeve, a roller, a control blade, a photosensitive drum or the like makes the charge control agent to come off from the surface of the toner, which may cause contamination of the carrier and so on, and contamination of a developing member, the photosensitive drum and so on. In this case, an increasing printing number of sheets leads to a decreased chargeability and results in progression of deterioration of peripheral members and causes problems such as a change in image density and a decrease in image quality. Therefore, a difficulty in coming off of the charge control agent from the toner must also be taken into consideration.
Further, to be applicable to full color toners, additives to the toners are preferably colorless, and further, to be usable for polymerization toners, the additives preferably have no polymerization inhibiting property.
As described above, many properties are required for charge control agents. Then, a charge control agent that can impart sufficient charges to the toner stably for a long period of time is demanded.
Further, processes for fixing toner images that have been developed include a pressure heating process utilizing a heat roller (hereinafter, referred to as “heat roller fixing process”) and a heat fixing process that involves fixing the toner images while contacting a sheet to be fixed with a heating member through a fixing film (hereinafter, referred to as “film fixing process”).
In the case of heat roller fixing process and film fixing process, the surface of the heat roller or fixing film is in pressure contact with the toner image on a sheet to be fixed. Therefore, the thermal efficiency of fusing the toner image on the sheet is extremely high. This allows rapid and good fixation to be performed.
In recent years, electrophotographic apparatuses are required to have various properties such as high image quality, compact size and light weight, high speed and high productivity, low energy consumption, high reliability, low cost, maintenance free property and so on. For the fixing process, development of a system and a material that can achieve higher speed, lower energy consumption, higher reliability is demanded. However, to solve those problems by the heat roller fixing process and film fixing process, it is indispensable to improve the fixing performance of the toner to a greater extent and it is necessary to improve low temperature fixing performance and also anti-offset performance.
Toners that contain wax having a high affinity with the binder resin exhibit good anti-offset performance and low temperature fixing performance under specified fixing conditions (see, for example, Japanese Patent Application Laid-Open Nos. H08-050367 and 2001-318484). However, in those toners, wax becomes compatible with the binder resin, so that the glass transition temperature of the toner and the melt viscosity of the toner tend to be decreased. In this case, if a further improvement of low temperature performance is attempted, chargeability in addition to storage stability and flowability tends to be deteriorated; in particular, when continuous printing is performed, a remarkable decrease in image density or a faulty image tends to occur. For this reason, a toner that has an excellent chargeability while it has an excellent low temperature fixing performance is demanded.
Incidentally, prevailing printer apparatuses include LED and laser beam printers in recent markets and a higher resolution is required in the technical trend. Further, copying machines are also in progress toward a higher function and hence toward digitalization. This direction of technology, which is mainly attained by a method of forming an electrostatic latent image by means of laser, also aims at a higher resolution. In addition, the developing method is required to be able to provide high precision images. As one means to meet the requirements, development of a toner with smaller particle size has been under way and toners having small particle sizes within specified particle size distributions (see, for example, Japanese Patent Application Laid-Open No. H01-112253, Japanese Patent Application Laid-Open No. H01-191156, Japanese Patent Application Laid-Open No. H02-284156, Japanese Patent Application Laid-Open No. H02-284158, Japanese Patent Application Laid-Open No. H03-181952, and Japanese Patent Application Laid-Open No. H04-162048) have been proposed.
However, the smaller the particle size of the toner, the more important it is to control the triboelectric charge of the toner. That is, in the case of the toner having a small particle size, failure to impart uniform charge quantity to individual toner particles results in a remarkable decrease in image stability. The reason will be as follows. (1) A toner having a smaller particle size has a greater affixing power (mirror image force, van der Waals force, etc.) to the photosensitive member, resulting in that the remaining toner after transfer tends to increase. (2) A decrease in toner particle size is accompanied by deterioration of flowability, so that the charge quantity of individual toner particles tends to be non-uniform, thus causing fog or a decrease in transferability.
Under the circumstances, a study for improving the charge properties of toner has been conducted intensively and in recent years, it has been proposed to use a resin having a charge control function as a raw material of toners because of consideration on environment, requirement for more stable chargeability, production cost, and so on (see, for example, Japanese Patent Publication No. H08-012467 and Japanese Patent No. 2663016).
According to the literatures, toners having an improved chargeability can be obtained. However, studies on the toners performed by the inventors of the present invention reveal that when the number of printouts becomes larger, toners of opposite polarity gradually increase in the developing device, which leads to a problem that so-called reversal fog tends to occur. Further, there is room for the toners to be improved for their low temperature fixability.
Further improved toners containing a copolymer of a sulfonic acid group-containing acrylamide and a vinyl monomer have been proposed (see, for example, Japanese Patent Application Laid-Open No. H11-184165, Japanese Patent Application Laid-Open No. H11-288129, and Japanese Patent Application Laid-Open No. 2000-056518). For the technologies disclosed in the literatures, however, there is room to be improved of charging performance (in particular, initial rise performance) when process speed is increased by a contact single-component development system or the like.
Conventionally, several proposals have been also made on the improvement of chargeability of the resin having a charge control function (see, for example, Japanese Patent Application Laid-Open No. H01-191156 and Japanese Patent Application Laid-Open No. H0-2284156). According to the literatures, toners showing relatively good rise in chargeability and having good dispersibility of various additives in the binder resin can be obtained. However, there is room for improvement of the stability of charge quantity before and after endurance running or under high temperature and high humidity and transferability.
Further, proposals have been made to use polyester resins having a sulfonic acid group as charge control resins (see, for example, Japanese Patent Application Laid-Open No. 2003-215853, Japanese Patent No. 2972987, and Japanese Patent No. 3179663). However, there is room for improvement of charging properties, and in particular, the stability of charge quantity under high temperature and high humidity conditions was insufficient.